COMMUNICATION pubs.acs.org/JACS
MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction Yanguang Li, Hailiang Wang, Liming Xie, Yongye Liang, Guosong Hong, and Hongjie Dai* Department of Chemistry, Stanford University, Stanford, California 94305, United States
bS Supporting Information ABSTRACT: Advanced materials for electrocatalytic and photoelectrochemical water splitting are central to the area of renewable energy. In this work, we developed a selective solvothermal synthesis of MoS2 nanoparticles on reduced graphene oxide (RGO) sheets suspended in solution. The resulting MoS2/RGO hybrid material possessed nanoscopic few-layer MoS2 structures with an abundance of exposed edges stacked onto graphene, in strong contrast to large aggregated MoS2 particles grown freely in solution without GO. The MoS2/RGO hybrid exhibited superior electrocatalytic activity in the hydrogen evolution reaction (HER) relative to other MoS2 catalysts. A Tafel slope of ∼41 mV/ decade was measured for MoS2 catalysts in the HER for the first time; this exceeds by far the activity of previous MoS2 catalysts and results from the abundance of catalytic edge sites on the MoS2 nanoparticles and the excellent electrical coupling to the underlying graphene network. The ∼41 mV/ decade Tafel slope suggested the VolmerHeyrovsky mechanism for the MoS2-catalyzed HER, with electrochemical desorption of hydrogen as the rate-limiting step.
H
ydrogen is being vigorously pursued as a future energy carrier in the transition from the current hydrocarbon economy.1 In particular, sustainable hydrogen production from water splitting has attracted growing attention.13 An advanced catalyst for the electrochemical hydrogen evolution reaction (HER) should reduce the overpotential and consequently increase the efficiency of this important electrochemical process.3 The most effective HER electrocatalysts are Pt-group metals. It remains challenging to develop highly active HER catalysts based on materials that are more abundant at lower costs.4 MoS2 is a material that has been commonly investigated as a catalyst for hydrodesulfurization.5 Recent work showed MoS2 to be a promising electrocatalyst for the HER. Both computational and experimental results confirmed that the HER activity stemmed from the sulfur edges of MoS2 plates, while their basal planes were catalytically inert.68 As a result, nanosized MoS2 with exposed edges should be more active for HER electrocatalysis than materials in bulk forms. Previously, MoS2 catalysts supported on Au,7 activated carbon,6 carbon paper,8 or graphite9 were prepared by physical vapor deposition or annealing of molybdate in H2S. Various overpotentials (from ∼0.1 to ∼0.4 V)10 and Tafel slopes (5560 mV/decade7 or > 120 mV/decade8) were reported. The mechanism and reaction pathways of the HER with MoS2 catalysts also remained inconclusive. r 2011 American Chemical Society
In recent years, our group has been developing syntheses of nanostructured metal oxide or hydroxide materials on graphene sheets, using either graphene on solid substrates or graphene oxide (GO) sheets stably suspended in solution.1115 These metal oxide or hydroxidegraphene hybrids are novel because of the chemical and electrical coupling effects and the utilization of the high surface area and electrical conductance of graphene, leading to advanced materials for nanoelectronics,11 energy storage devices (including pseudocapacitors13 and lithium ion batteries14), and catalysis.15 Here we report the first synthesis of MoS2 on reduced graphene oxide (RGO) sheets and demonstrate the high HER electrocatalytic activity of the resulting MoS2/RGO hybrid with low overpotential and small Tafel slopes. The MoS2/RGO hybrid was synthesized by a one-step solvothermal reaction of (NH4)2MoS4 and hydrazine in an N,Ndimethylformamide (DMF) solution of mildly oxidized graphene oxide (GO; see Figure S1 in the Supporting Information)14 at 200 °C (Figure 1A; nominal C/Mo atomic ratio ∼10; see the Supporting Information for synthetic details). During this process, the (NH4)2MoS4 precursor was reduced to MoS2 on GO and the mildly oxidized GO transformed to RGO by hydrazine reduction.16 Figure 2A,B shows scanning electron microscopy (SEM) images of the resulting MoS2/RGO hybrid, in which the RGO sheets were uniformly decorated with MoS2 nanoparticles. The transmission electron microscopy (TEM) image (Figure 2C) shows that most of the MoS2 nanoparticles lay flat on the graphene, with some possessing folded edges exhibiting parallel lines corresponding to the different layers of MoS2 sheets (number of layers = 310; Figure 2C inset). High-resolution TEM revealed hexagonal atomic lattices in the MoS2 basal planes and abundant open edges of the nanoparticles (Figure 2D). The MoS2/RGO hybrid was characterized by X-ray diffraction (XRD), and the broad diffraction peaks (Figure 2E) indicated nanosized MoS2 crystal domains with hexagonal structure [powder diffraction file (PDF) no. 771716]. Raman spectroscopy revealed the characteristic peaks17 of MoS2 at 373 and 400 cm1 and the D, G, and 2D bands of graphene in the hybrid (Figure 2F). The uniform distribution of MoS2 on RGO was confirmed by micro-Raman imaging of the two components in the hybrid deposited on a substrate (Figure S2). X-ray photoelectron spectroscopy (XPS) confirmed the reduction of GO to RGO and Mo(VI) to Mo(IV)18 (Figure S3). The residual oxygen content in the hybrid was measured to be
